Anyway, here's a circuit to try. It uses an L293 and a 4093 as the active components.The reverse delay is made by R1/C1 for the left side and R6/C3 for the right side.

The LED's and their resistors is just meant as a test aid and can be removed when it works. LED1 and LED4 could be green, for easier differentiation. They show which direction the motor will turn.

The bumper switches (attached to SW1+, SW1-, SW2+ and SW2-) could be made of piano wire that touches a contact point when hitting a wall. It will have to cover each front "corner", so that both a direct hit and a slanted hit will make contact.

You can use a 4011 instead (in this app), the only difference is that it haven't got Schmitt trigger inputs, so it will be a little harder to calculate R1/C1 for the reverse time, but with a bit of trial and error, you should be able to get there.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives